As discussed in the afore-mentioned patent application Ser. No. 209,629, in an Integrated Services Digital Network (ISDN), as defined in the recommendations of the International Telegraph and Telephone Consultative Committee (CCITT), and ISDN telephone subscriber loop includes a U-interface and a T-interface. The U-interface loop includes a two-wire full duplex digital signal transmission line, and extends from a telephone central office or exchange termination (ET) to a network termination (NT). The network termination couples the signals between the U-interface and the T-interface, which includes a four-wire digital signal transmission line for providing transmit and receive signal paths between the network termination (NT) and the terminal equipment (TE), usually including at least one, and as many as eight, ISDN telephone sets.
The network termination (NT) must be able to correlate the information being received on the receive signal path, from the terminal equipment (TE), with the information which it has just transmitted to the TE on the transmit signal path.
It is desirable to have a network termination timing recovery arrangement which is able to accommodate the different configurations envisaged by ISDN T-interface standards, and which will be able to extract the timing despite the different mean phases of the signals received from plural terminal equipments (TE).
Recommended configurations and operating parameters for the T-interface are defined in the CCITT Red Book, layer 1 specification I.430, published 1985, updated 1986, and American National Standard T1.XYZ.1918Y. (ANSI specification), in the latter document specifically at Section 8.6.3. These recommendations entertain provision of up to four kinds of Network Termination to support the various T-interface configurations. Those specified are for Short Passive Bus, Point-to-Point, Extended Passive Bus, and both Point-to-Point and Passive Bus. The round trip delay for Point-to-Point and Extended Passive Bus configurations ranges from 10 to 42 microseconds. For the Short Passive Bus, the round trip delay is in the range of 10 to 14 microseconds, and for the combination of Point-to-Point and Passive Bus, the round trip delay should be in the range of 10 to 13 microseconds for Passive Bus and 10-42 microseconds for Point-to-Point. In the case of the Extended Passive Bus, the differential delay between signals from different termination equipments is in the range 0 to 2 microseconds.
Each Network Terminal (NT) will synchronize its signal transmitted on the transmit path of the T-interface with the signal it is receiving on the U-interface. The difficulty lies in synchronizing the signals being received on the receive path of the T-interface because they will have been originated by different terminal equipments, and so will have different amounts of jitter and phase shift.
In a so-called fixed timing recovery system, the network termination identifies the beginning of the frame being transmitted, delays a predetermined length of time (typically less than a bit period) and then samples the incoming receive path for the corresponding receive signal. This arrangement is not entirely satisfactory, however, because it arbitrarily fixes the maximum loop length to less than the bit period.
Adaptive timing recovery systems have been disclosed in which the sampling instant is varied with respect to a predetermined reference, namely a zero-crossing, in the digital signal received from the terminal equipment. A disadvantage of such adaptive timing recovery systems is that the positions of the zero-crossings in different signals differ according to the varying distances between the network terminator and the terminal equipment.
The problem of adapting timing recovery for different configurations has been addressed by Yasuyuki Okumura, Kazuhiro Hayashi, and Yuji Inoue in a paper entitled "A New Phase Locked Oscillator Adaptable to Input Signals With Periodical Phase Jumps" Proceedings of ISCAS 85, IEEE; by Yasuyuki Okumura, Takashi Yamamoto, and Masasha Kuribayashi, in a paper entitled "Circuit Design and Transmission Performance for ISDN Basic Interface", IEEE, 1986; and by Yasuyuki Okumura and Kazuhiro Hayashi in U.S. Pat. No. 4,682,327 entitled "Polyphase Phase Lock Oscillator", issued July 21, 1987, all of which are incorporated herein by reference. They proposed an adaptive timing extraction method using a polyphase phase-locked oscillator. This oscillator comprises a phase-locked loop for each individual channel which extracts the timing clock pulse from the channel independently, following segregation of the input signals for each channel by gate signals generated using the marker from the received signal.
This arrangement is not entirely satisfactory because it presumes that each channel will be allocated to a single terminal equipment and moreover requires complex circuitry.
Canadian patent number 1,242,502 issued Sept. 9, 1988 and incorporated herein by reference, discloses an adaptive rate recovery circuit for receiving digital data. The adaptive rate recovery circuit uses two clocks to provide two alternative sampling instants spaced apart by a predetermined amount. A quadratic function relating the two timing instants is computed and the actual timing instant determined according to the sign of the quadratic function. This is not entirely satisfactory since it employs two clocks and is complicated.
The present invention seeks to ameliorate the problems associated with such known systems.